1. Field of the Invention
This invention relates to an improved method for electrodialysis of salts of weak acids and/or weak bases. More particularly; the invention relates to a method used to obtain high purity base from the salt of a weak acid such as sodium formate; or high purity acid from the salt of a weak base, such as ammonium nitrate.
2. Description of Related Art
Electrodialysis uses direct current as a means to cause the movement of ions in solutions. Electrodialysis processes are well known in the art and are typically carried out in a stack arrangement comprising a plurality of flat sheet membranes. The stack consists of electrodes (anode and cathode) at either end and a series of membranes and gaskets which are open in the middle to form a multiplicity of compartments separated by the membranes. Usually, a separate solution is supplied to the compartments containing the electrodes. Special membranes may be placed next to the electrode containing compartments in order to prevent mixing of the process streams with the electrode streams. The majority of the stack between the electrode compartments comprises a repeating series of units of different membranes with solution compartments between adjacent membranes. This repeating unit is called the unit cell, or simply, a cell. Solution is typically supplied to the compartments by internal manifolds formed as part of the gaskets or by a combination of internal and external manifolds. The stacks can include more than one type of unit cell. Streams may be fed from one stack to another in order to optimize process efficiency. Usually the change in composition of a stream after one pass through the stack is relatively small and the solutions can be recycled by being pumped to and from recycle tanks. Addition of fresh solution to and withdrawal of product from the recycle loop can be made either continuously or periodically in order to control the concentration of products in a desired range.
Treatment of aqueous salt streams by electrodialysis to form acid and/or base from the salt is known. The aqueous salt stream is fed to an electrodialytic water splitting apparatus which comprises an electrodialysis stack and a means for electrodialytically splitting water. A useful apparatus is disclosed in U.S. Pat. No. 4,740,281. A useful means to split water to (H.sup.+) and (OH.sup.-) is a bipolar membrane such as disclosed in U.S. Pat. No. 4,766,161. The bipolar membrane is comprised of anion selective and cation selective layers of ion exchange material. In order for the membrane to function as a water splitter, the layers must be arranged so that the anion layer of each membrane is closer to the anode than the cation layer. A direct current passed through the membrane in this configuration will cause water splitting with hydroxyl ions being produced on the anode side and a corresponding number of hydrogen ions being produced on the cathode side of the membrane. The disassociated salt anions move toward the cathode and the disassociated salt cations move toward the anode.
Electrodialytic water-splitting in a two-compartment cell has been disclosed, for example, in U.S. Pat. No. 4,391,680 relating to the generation of strongly acidified sodium chloride and aqueous sodium hydroxide from aqueous sodium chloride. U.S. Pat. No. 4,608,141 discloses a multi-chamber two-compartment electrodialytic water splitter and a method for using the same for basification of aqueous soluble salts. U.S. Pat. No. 4,536,269 disclose a multi-chamber two-compartment electrodialytic water splitter and a method for using the same for acidification of aqueous soluble salts. These two patents review the use of two-compartment electrodialytic water splitters to treat salts.
Three-compartment electrodialytic water splitters are disclosed to be comprised of alternating bipolar, anion and cation exchange membranes thereby forming alternating acid, salt and base compartments. U.S. Pat. No. 4,740,281 discloses the recovery of acids from materials comprising acid and salt using an electrodialysis apparatus to concentrate the acid followed by the use of an electrodialytic three-compartment water splitter to separate the acid from the salt.
The purity of the acids and bases produced from salts by water-splitting is sometimes inadequate. A major source of contamination of the acid and base results from the transport of anions from the acid to the base and cations from the base to the acid. These processes are described in Sirkar et al., Editors, New Membrane Materials and Processes for Separation; Chlanda et al., Water Splitting Efficiency of Bipolar Membranes, AIChE Symposium Series 1988 and the presentation of T. A. Davis, "Coion Transport in Bipolar Membranes", American Chemical Society, 1986 Southwest Regional Meeting, Nov. 20, 1986. The problem becomes even more severe when weakly ionized acids and bases are being produced. A particular source of contamination is the diffusion of the nondisassociated acid or base across the membranes. These transport processes which result in contamination of the acid and/or base products are illustrated in FIG. 1.
Means to purify acids and bases by the use of bipolar membranes have been described in U.S. application Ser. No. 278,062, filed Dec. 1, 1988, and U.S. Pat. No. 4,976,838. These processes are general to acids and bases obtained from any sources, including the water-splitting operation.